Predict method allows bonehead null-reference bug

[normanhh3]

Hi folks,

Great work here!

I've recently run into a null-reference exception on this line due to not providing a proper Descriptor. I realize that is clear when reading the code, it just isn't all that clear from creating a model and then loading it from a serialized state and oh, you also need a descriptor to go with it.

Anyway, a pre-condition check on the Descriptor would, I think help clarify things for new folks.

[sethjuarez]

Good call